1. Field of the Invention
This invention relates to organosiloxane compositions. More particularly, this invention relates to improved organosiloxane compositions that can be cured by a platinum-catalyzed hydrosilation reaction to form elastomers exhibiting superior physical properties relative to prior art elastomers, particularly with respect to their ability to recover a larger percentage of their initial thickness following prolonged compression at temperatures above 100.degree. C. This is achieved without sacrificing other desirable properties such as processability of the curable composition and without the need for post-curing. Preferred compositions are extrudable.
2. Description of the Prior Art
Curable organosiloxane compositions comprising at least one polydiorganosiloxane with at least two silicon-bonded vinyl radicals, an organohydrogensiloxane, a platinum catalyst and a silica filler are well known in the art pertaining to organosiloxane elastomers. It is also known to add peroxides to certain compositions containing a platinum-based catalyst and hydroperoxide as a catalyst inhibitor to accelerate curing at temperatures above about 100.degree. C.
Published British patent application no. 2,049,717 that issued to William Bobear on Dec. 31, 1980 teaches curable organosiloxane elastomer compositions comprising a vinyl-substituted organopolysiloxane, an organohydrogenpolysiloxane, a platinum-containing hydrosilation catalyst, a hydroperoxide to inhibit curing of the composition at room temperature and an organic peroxide to accelerate curing at temperatures above 100.degree. C. Bobear teaches that any organic peroxide will function as a curing accelerator in combination with the hydroperoxide inhibitor. The effect of the organic peroxide on the recovery properties, particularly recovery from long-term compression, is not discussed in this publication.
Organic peroxides can be classified into two categories with respect to the mechanism by which they cure polyorganosiloxanes. One category of peroxides generate acyloxy radicals and function by abstracting hydrogen atoms from any of the silicon-bonded hydrocarbon radicals of the organopolysiloxane, irrespective of whether the hydrocarbon radicals contain terminal ethylenically unsaturated carbon atoms. This category of peroxides are typically referred to as "non vinyl specific", and include benzoyl peroxide and 2,4-dichlorobenzoyl peroxide.
The second category of organic peroxides generate principally tertiary alkyl radicals as initial decomposition products, and are considerably more selective with respect to their ability to abstract hydrogen atoms from hydrocarbon radicals containing terminal ethylenic unsaturation. Using this type of peroxide the number of crosslinks generated during curing approaches the number of vinyl or other terminally unsaturated hydrocarbon radicals present in the curable organosiloxane cmmposition. Vinyl-specific peroxides include t-butyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane and 2,5-dimethyl-2,5-bis(t-butylperoxy)hexyne-3.
U.S. Pat. No. 3,843,601, which issued to L. Broner on Oct. 22, 1974 contains a detailed discussion of vinyl-specific and non-vinyl specific peroxides used as curing agents for organosiloxane compositions.
U.S. Pat. No. 4,329,275, which issued to Hatanaka et al. on May 11, 1982 discloses heat curable polysiloxane compositions comprising a polyorganosiloxane containing at least two vinyl radicals per molecule, a polyorganohydrogensiloxane, a platinum-containing catalyst, a phosphorus compound and an organic peroxide. The platinum catalyst and the phosphorus compound can be present in the form of a complex, and the organic peroxide can be of the type referred to as vinyl-specific, which includes aliphatic peroxides such as 2,5-dimethyl-2,5-di(t-butylperoxy)hexane or non-vinyl-specific peroxides such as 2,4-dichlorobenzoyl peroxide. The concentration of peroxide is limited to less than 0.1 part by weight per 100 parts of the vinyl-containing polydiorganosiloxane, and its effect is to control the activity of the platinum catalyst at ambient temperature without detracting from the activity of this catalyst at the higher temperatures used to cure the composition.
Organosiloxane compositions containing organic peroxides as the curing agent typically must be cured by heating them in a confined area such as a mold to prevent premature volatilization of the peroxide with resultant incomplete curing of the organosiloxane composition. This technique is not feasible for those applications where an extrudable organosiloxane composition is applied to the surface of a substrate and allowed to cure under ambient conditions or with the application of heat.
For some end use applications a cured organosiloxane elastomer is intended to function as a seal between the surface of an article to which the curable organosiloxane precursor is initially applied and a surface of a second article which is placed in contact with the cured elastomer. To maintain the integrity of such a seal the dimensions and recovery properties of the cured elastomer layer must not change appreciably with time, even when heated. The extent to which an elastomeric article recovers its initial thickness after being compressed with heating for periods of up to 24 hours or longer is referred to as compression set.
Elastomers prepared from prior art extrudable organosiloxane compositions are typically cured by heating them for five minutes or less at temperatures from 100.degree. to about 200.degree. C. The compression set values for these elastomers are typically greater than 40 percent, measured in accordance with ASTM test procedure D 395. In other words, the difference between the initial thickness of a test sample molded from the elastomer and its thickness following the test is at least 40 percent of the difference between the initial thickness and the thickness to which the sample is compressed during the test.
In accordance with one embodiment of the aforementioned ASTM test method an elastomer sample is compressed to 75 percent of its initial height for a period of 70 hours while heating it at temperatures above about 150o C. The formula for calculating compression set is 100 x (a-b)/(a-c), where a is the initial thickness of the test sample, b represents the thickness of the test sample following completion of the test and c represents the thickness to which the sample is compressed during the test.
To be suitable for use as gaskets and other types of seal-forming materials the compression set value of the cured elastomer should be about 20 percent. Compression set values in this range can typically be achieved only by post-curing, which requires heating the cured elastomer for from 30 to 60 minutes at temperatures of about 200.degree. C.
An objective of this invention is to define a class of liquid organosiloxane compositions that can be rapidly cured to yield elastomers exhibiting the combination of high tear and tensile strengths in addition to compression set values of less than 30 percent, measured using the procedure of ASTM test method D 395. A preferred class of the present compositions are "extrudable", as defined by their ability to be extruded at a rate of at least 45 grams per minute through a 3.175 millimeterdiameter orifice under a pressure of 620 kilopascals (kpa) at 25 degrees C.